Hi,
at http://www.w3.org/TR/mathml-units/ section 2 "Display of Units in
Presentation MathML" first table "MathML Source" second row:
<mi mathvariant='normal' class='MathML-Unit'>J</mi> <mo>&middot;</mo>
<msup><mi mathvariant='normal' class='MathML-Unit'>kg</mi>
<mn>-1</mn></msup><mfrac>
The <mfrac> at the end is false. You can take it away.
Bye
Alexander Kiel